A motor is an electromagnetic apparatus for conversion of electromechanical energy based on the electromagnetic reaction principle. A Permanent Magnet Synchronous Motor (PMSM) acts as an increasingly important part in modern industries and household products for its relatively higher power density and electric efficiency.
The electromagnetic structure of a motor is normally designed with a radial magnetic field structure, as shown in FIGS. 1 and 2. In FIGS. 1 and 2, the basic structure of a PMSM having an outer rotor is shown, which comprises a rotor housing 1′, a rotor permanent magnetic 2′, a stator core 3′ and an armature winding 4′ wound in the stator core 3′. In FIG. 3, an electromagnetic structure of this motor is shown, in which an N pole and an S pole of the rotor permanent magnet 2′ are alternatively mounted on the rotor housing 1′.
In the case of the motor structure having a radial magnetic field as shown in FIGS. 1 and 2, the structure of the armature winding is required to be 3-dimensional, as shown in FIG. 3. A 3D winding is formed by using a winding machine, or wounding manually. As such, the dimensions of the winding cannot have a high accuracy.
Alternatively, a motor may also be designed with axial magnetic field structure. Some motor structures using an axial magnetic field may have 2D structures of armature winding. As shown in FIG. 4, such motor structure has a rotor yoke 1″, a rotor permanent magnet 2″, a stator yoke 3″, an armature 4″ between the stator yoke 3″ and the rotor permanent magnet 2″, and a rotor base. This winding is generally a fractional concentrated winding. A three-phase winding is arranged in different places on a same layer in a 2D space, the winding of each phase is not overlapped with the others. This winding may be made by bonding wires, or even Printed Circuit Board (PCB) technique, thus reaching a higher precision in its dimensions. The precision of the winding plays an important role in the motor having an axial magnetic field since it relates to the size of an air gap of the motor and thus affects the performance of the motor.
For the existing axial magnetic field motor, the permanent magnet on the rotor is arranged in space forming an electrical angle of 180 degrees while the span of the fractional concentrated winding in space is less than 180 degrees (e.g. 120 degrees), the use efficiency, in which the winding couples the magnetic field generated by the magnet on the rotor is relatively low. This also makes the motor below in power density and motor constant.
In theory, a 2D winding may also use a span of 180 degrees to improve the use efficiency in utilizing the magnetic field generated by the rotor. However, such winding leads to overlap in windings of the different phases. In this case, the three-phase winding cannot be achieved at one PCB layer. The arrangement, where the three-phase winding must be located on different layers, will induce significant asymmetry with respect to the magnetic performance of the three-phase winding. During the design and manufacturing of the PCB, windings of a same phase located on different layers have to be connected with “blind hole”, or “buried hole”. This significantly increases the cost of the PCB.
Additionally, if the winding is on different layers, the A-phase winding is closest to the rotor permanent magnet and the C-phase winding is farthest from the rotor permanent magnet. Thus, in the case of a same number of turns, the magnetic field of the A-phase winding is much higher than that of the C-phase winding. Therefore, when the motor rotates, the electromotive force generated in the A-phase winding is much greater than that of the C-phase winding. This will lead to a heavy asymmetry of the electromotive forces in the three phase windings, and this is not allowed in many applications.